Principle of Vector Network analyzer

Network Analyzer composition block diagram

Figure 1 shows the internal block diagram of the network analyzer. To test the transmission/reflection characteristics of the tested parts, the network analyzer is included;

1. Excitation signal source, supply of the measured part excitation input signal

2. The signal separation device, including the power divider and the directional coupling device, respectively extracts the input and the reflected signal of the tested part.

3. The receiver is tested by the reflection, transmission and input signal of the measured part.

4. Processing the display unit; The test results are processed and displayed.

Figure 1 Network analyzer composition diagram

The transmission characteristic is the relative ratio between the output of the measured part and the input excitation, and the network analyzer needs to get the input excitation signal and output signal information of the measured part separately to complete the test.

The internal signal source of the network Analyzer is responsible for generating the excitation signal which satisfies the test frequency and power requirement, and the signal source output is divided into two signals through the splitter, which goes directly into the R receiver and the other way through the switch input to the corresponding test port, so the R receiver is tested to get the input signal information.

The measured part output signal enters the network analyzer b receiver , so theb receiver tests the output signal of the measured part. The B/R is the positive transmission characteristic of the tested piece. When the reverse test is complete, the Network Analyzer internal switch control signal flow is required.

Figure 2 Network analyzer transmission test signal flow

The reflection characteristic is the relative ratio between the measured part reflection and the input excitation, and the network analyzer is required to obtain the input excitation signal and the test port reflection signal respectively to complete the test.

The internal signal source of the network Analyzer is responsible for generating the excitation signal which satisfies the test frequency and power requirement, and the signal source output is divided into two signals through the splitter, which goes directly into the R receiver and the other way through the switch input to the corresponding test port, so the R receiver is tested to get the input signal information.

When the excitation signal is input to the test piece, the reflected signal and the input excitation signal are transmitted on the same physical path, and the directional coupler is responsible for separating the signal transmitted in the opposite direction on the same physical path, extracting the reflected signal information and entering the a receiver.

The A/R is the reflective characteristic of the tested piece port. When additional port reflection characteristics need to be tested, the network Analyzer internal switch is required to convert the excitation signal to the appropriate test port.

Figure 3 Network Analyzer reflection test signal flow

Signal source

The signal source provides the excitation signal of the tested piece, as the network analyzer is to test the relationship between the transmission/reflection characteristics of the measured parts and the operating frequency and power. Therefore, the signal source in the network analyzer must have frequency scanning and power scanning function.

In order to ensure the frequency accuracy of the test, the frequency synthesis method is used to realize the signal source in the network analyzer. When the sweep width is set to zero, the output signal is a point-frequency CW signal.

Network analysis control its output power depends on the ALC and attenuator two parts completed. The ALC guarantees the stability of the input signal power and the power scanning control, because the ALC control range is limited, the attenuator is required to complete a wide range of power regulation

Figure 4 Signal sources in the network analyzer

Signal Separation Device

The internal power divider and directional coupler of the network analyzer are respectively completed to extract the input signal and the reflected signal of the measured part. When you want to test a port reflection characteristic of a tested piece, the directional coupler must be connected directly to the test port.

These two parts are collectively referred to as the signal separation device, this part of the hardware is usually tested as "test seat", in some special test occasions (high power testing, etc.) can not use the Integrated network analysis instrument built-in test seat, and the use of external test seat equipment.

Fig. 5 Signal separation device in Network analyzer

The bridge is used for reflection performance testing, the bridge can cover a wide frequency range, the main disadvantage of the bridge is the large loss of transmission signal. So for a given signal source power. The resulting power loss is caused by the input to the test piece.

The directional coupler is responsible for separating the excitation and reflection signals from the reflection test, which can also be done by the bridge, which can cover a wider frequency range compared with the directional coupler, which has a greater loss of transmission signal to the test.

The directional coupler is a three-port device whose three ports are input, output and coupling.

In the reflection test, the directional coupler is needed, which is the directional transmission characteristic using directional coupling. When the signal is connected by the input of the directional coupler, the coupling end has a coupling output, which is called forward transmission, the directional coupler is equal to the non-evenly divided power splitter.

In forward transmission, the coupler coupling output is defined as the ratio of the input power to the coupling.

Fig. 6 Forward transmission characteristics of the directional coupler

For the ideal directional coupler, the coupling end is not output when the signal is operated by the coupler output side in reverse. This is because the input power is absorbed by the internal load of the coupler and the external load of the main arm terminal, which is the unidirectional transmission of the directional coupler.

When the actual directional coupler is in reverse operation, the coupling end will have leakage output, and the coupling end output and the input signal power ratio are defined as the directional coupler isolation degree when working backwards.

Figure 7 Directional coupler Reverse transmission characteristics

The important index of directional coupler test is its directivity (directivity), and the directivity is the difference between the inverse working isolation degree and the positive working coupling degree of the directional coupler. The directivity indicator reflects the ability of the coupler to separate the positive and negative two directional signals. Can be considered as the dynamic range of the reflection test.

There is an easy way to measure directional couplers without the need for forward and reverse connection tests. When the internal load loss power of the directional coupler is quite an hour, the result of this method is similar to the real value.

First, a short-circuit load is terminated on the main arm output, and due to full reflection, the coupling output reflects the coupling degree, and the value is regulated and then terminated to match the load. At this point the coupling end is only the leakage signal caused by the limited isolation degree. Because the rules have already been processed, the last read value is the coupler directionality.

In the reflection test, the directional coupler is a positive connection to the reflected signal of the measured part, and the output of the directional coupler coupling end reflects the reflected signal information.

When the network analyzer tests the reflective characteristics, the coupling end of the coupler contains the leaked input excitation signal, which is superimposed with the reflected signal, resulting in the measurement error of the reflection index due to the limited directional influence of the directional coupler.

The better the matching performance of the tested parts, the more influence the directional coupler directivity has on the test.

Receivers in a network analyzer

The signals from the power divider, the directional coupler and the output end are input to the corresponding receivers for processing, and the network Analyzer is built into several receivers for the analysis of these signals.

The network analyzer is a closed-loop test system with excitation source and receiving equipment.

Figure 8 Network Analyzer Receiver

There are two basic methods of detecting signal in Network analyzer.

Method 1: Diode detection, diode detection to extract the RF signal input envelope level, the output voltage reflects the input signal power. If the input signal is a continuous CW signal, the DC detector, if the input is a amplitude modulated signal, for AC detection. The diode detector only reflects the signal amplitude information and loses the phase information of the RF carrier signal.

Method 2: Tune the receiver. The tuned receiver converts the input signal into a digital post-processing via the ADC after conversion. This gives the signal phase and amplitude signals.

If you use a power meter, you will understand the characteristics of the detector signal. First the detector is a broadband power test, both if the detector operating frequency range is 10M to 18G, and its power display should be the frequency range of all the signal power, and no frequency selection test function. For this reason, a scalar network analyzer using a detector will have no ability to differentiate between distortion and clutter at the output of the tested piece, and will result in incorrect test results.

But the scalar Network Analyzer uses the same method to test the variable frequency and non-variable parts.

The range of power the detector can detect is limited, for example, -50DBM~10DBM, which limits the dynamic range of scalar Network analyzer tests.

Tuned receiver due to the frequency signal to pass through the band-pass filter processing, due to the detector bandwidth test mode, this no-selection test will result in large test noise bandwidth (20G), and the tuning receiver if the bandwidth can be as small as 1KHz, so that the receiver has a good test sensitivity, It also has a good inhibitory effect on the clutter distortion component in the output signal of the measured part.

The sensitivity of the tuned receiver is directly related to the bandwidth of the intermediate frequency filter, the narrower the band width, the less noise energy enters the receiver, the sensitivity increases, but the response time of the output signal becomes longer, and the test speed of the network analyzer is decreased.

Narrowband Receiver Network Analyzer if the bandwidth of the test is one of the basic parameters, its setting value is a tradeoff between the test accuracy and speed.

Fig. 9 Tuning receiver and its characteristics

This is the comparison between the test results of the detector and the tuned receiver, respectively, using the same tested piece.

example, the test piece is a filter, when the filter out-of-band rejection performance testing, at this time, the Network analyzer output excitation signal by the filter's inhibition into a small signal,

Filter output = input signal Power 0dBm-100db= -100dbm with out-of-band rejection of the waveform.

If the detector sensitivity is -60dbm, the -100dbm actual signal can not be detected, the test result can not reflect the measurement parameters.

Compared with the detector, the tuned receiver has a very small detection bandwidth, which detects high sensitivity and can actually get the tested parts.

A vector network analyzer using a tuning machine can extend the measurement dynamic range by increasing the output power, decreasing the if bandwidth, or using the averaging function (avg) .

Figure 10 The impact of the dynamic range of the network analysis table on the test results

Agilent PNA Series network analyzers are high-performance vector network analyzers with tuned receivers. The receiver test has high sensitivity and can meet the dynamic range requirements of various test parts.

The tuner receiver can use the mixer and sampler two ways to realize the function of the front-end converter.

Sampler (Sampler) is the use of two-stage tube to the input RF signal by pulse extraction process, the sampler can be considered as the internal pulse generator mixer, the pulse generator generated by the local oscillator harmonic composition of the wideband spectrum (comb spectrum), the input RF signal and comb line one of the signal mixing frequency output.

The local oscillator signals required by the frequency converter circuit of the sampler need only cover the narrow frequency range, the disadvantage of which is to lock the different comb spectrum lines to carry on the complex phase-locked processing, and compared with the mixing circuit, the noise of all the comb-loaded spectral lines will be transformed into the intermediate frequency signal, the sensitivity is worse.

Network Analyzer is a comprehensive excitation and reception of the closed-loop test system, the use of narrow-band tuned receiver Vector Network Analyzer work, the signal source generates an excitation signal, the receiver should be at the same frequency to the measured part response signal processing, the excitation source and receiver operating frequency changes should be synchronous change. The network analyzer relies on the phase-locked method to accomplish this function.

R Channel Receiver If the signal will be with the fixed reference signal phase, phase-contrast error output for the voltage control change the excitation source output frequency, so that when the receiver of the local oscillator frequency scanning changes, the phase-locked loop will control the excitation source to maintain frequency synchronization changes. When the R channel receiver is not working properly, the network analyzer will appear out of lock phenomenon.

Principle of Vector Network analyzer